1. Field of the Invention
The method of the present invention relates to the field of seismic data interpretation for the purpose of finding natural occurrences of oil and/or gas in a geological formation. The invention relates to a method of processing seismic geophysical data to produce time structure volumes. The method of the present invention is capable of displaying the position and orientation of layered rocks in the subsurface of the earth.
2. Description of the Prior Art
Seismic data are produced by transmitting an acoustic signal into the Earth and recording echoes of this signal. Various means are used in the prior art to produce an acoustic signal, such as dynamite. The Earth typically consists of layers of rock, and the acoustic properties of the rock typically change from layer to layer. Echoes are caused by changes in acoustic properties, and surfaces that produce echoes constitute xe2x80x9csurfaces of reflectionxe2x80x9d. The geometric relationships of these layers, as expressed by the surfaces of reflection, are collectively known as the xe2x80x9cstructure of the subsurfacexe2x80x9d.
It is desirable for interpreters to know whether the layers are tilted and the degree of tilt, and whether they form curved rather than planar surfaces, and whether the layers are broken or discontinuous. Subsurface rocks are generally porous, like beach sand but with less ability to absorb fluids, and contain either water, oil, or gas. Oil and gas are lighter than water and float upward in the subsurface as they do at the surface. The path of movement and the cessation of movement is in large part dependent on the geometry of the subsurface layers in which the fluids move, which makes this geometry of interest to seismic interpreters.
The geometry also allows the interpreter to deduce various geological characteristics, including how the rocks were deposited This history can be important in more sophisticated analyses regarding the generation of oil and gas and the distribution of pores in the subsurface.
To detect geometric relationships, echoes must be collected over an area, so seismic receivers are typically laid out along a line or in a grid pattern over the surface of the Earth. Prior art methods employ arrays of acoustic receivers to collect these echo signals. Capture over an area permits comparison of echoes from location to location. Prior art methods record echo signal amplitudes for each receiver as a function of time. These signals are knows as xe2x80x9cecho trains.xe2x80x9d Prior art echo trains may display time on a vertical axis and amplitude on a horizontal axis.
When echo trains from the grid locations are organized for display as a whole, the collection is referred to a xe2x80x9cseismic volumexe2x80x9d because it is the seismic representation of a volume of subsurface rock. FIG. 1 depicts a typical seismic trace as detected by a receiver at the surface. In FIG. 1, time is depicted on the vertical axis in seconds. Amplitude is shown on the horizontal axis. The strength of the echo rises and falls over a period of several seconds, and this rise and fall with time is recorded for processing and analysis. A single echo train is referred to as a xe2x80x9cseismic tracexe2x80x9d.
There are sophisticated techniques used in the prior art for improving the signal quality of seismic traces, such as by averaging echo trains from several receivers. xe2x80x9cInterpretationxe2x80x9d of seismic data refers to deducing the possible existence and location of oil and gas in the volume of rock represented by the seismic data. Prior art methods have employed the study of xe2x80x9clinesxe2x80x9d of seismic data. A line consists of data from one row or column of the grid of receivers described above. A line display provides a profile view of the seismic echoes so that one can readily see differences in the chain of echoes along the line.
FIG. 2 depicts a typical seismic line, and FIG. 3 shows in map view the location of this line in a grid of lines. A seismic volume consists of the collection of all the seismic lines or all of the seismic traces. The seismic volume may be sliced at various angles to display information of interest. A vertical slice through a seismic volume results in a display of amplitude data as a function of time for a line, as shown in FIG. 2. A vertical line intersects the dotted horizontal axis near the top of FIG. 2. The trace shown in FIG. 1 is also depicted below this vertical line in FIG. 2. For background purposes, FIG. 3 shows a typical grid of seismic data lines in map view. The location of every tenth north-south and east-west line is drawn in map coordinates. The location of the east-west section shown in FIG. 2 is indicated by the heavy dashed line in FIG. 3.
A horizontal slice through the seismic volume is referred to as a xe2x80x9ctime slice.xe2x80x9d A time slice depicts amplitude data as a function of spatial position, as shown in FIG. 4. FIG. 4 depicts a horizontal slice across each of the vertical sections at a time of 2.758 seconds. The circle traces in FIG. 2 correspond to the circled area shown in FIG. 4. By comparing FIGS. 2 and 4, one can see that the dark amplitudes of FIG. 2 at 2.758 seconds are part of sinuous north-south trend. Vertical and horizontal slices through the seismic volume to display amplitude data as a function of time and spatial position, respectively, have been employed in the prior art.
As seen in FIG. 4, the data display is blurred and lacks crisp detail. This lack of detail translates to a lack of precise information regarding the structure of the subsurface. The present invention provides an improved method of processing and displaying seismic volume data to enhance structural resolution.
The present invention provides a method for making structural relations easily seen in areal view. The present invention provides an enhanced understanding of the structure of the subsurface formation without interpreting data line by line. Application of the invention may save significant time and improve accuracy as well as thoroughness in the search for oil and gas. The invention is described in terms of seismic volumes when the vertical measurement is time. The present invention also applies when the vertical axis is measured in depth. If the vertical measurement is depth, the present invention uses depth slices, instead of time slices. Accordingly, in the present description of the invention, the term xe2x80x9ctimexe2x80x9d can be replaced with xe2x80x9cdepthxe2x80x9d where the vertical axis is measured in depth.
The present invention converts the information contained in a three dimensional volume of seismic trace data to multiple arrays of time structure values which can be collectively displayed as a time structure volume to more accurately and clearly depict the structure of the geological formations from which the seismic trace data was obtained.
A time structure display resulting from practicing the present invention is shown in FIG. 5. The amplitude information of FIG. 4 is replaced by structural information in FIG. 5. FIG. 4 shows the amplitudes of traces where they intersect the time slice, and the display has the same general lower left to upper right trend as is shown in FIG. 5; however, FIG. 4 lacks the structural detail and crispness of FIG. 5. Independent of its crispness, FIG. 4 does not show events (characteristic changes in amplitude, such as peaks or troughs), or where events intersect a time slice, or the direction in which an event is to be found below the time slice. FIG. 4 gives an indication of the geophysical response of rock at a given time slice; however, it does not illuminate the structure of the geology. The time slice view represented by FIG. 4 does not convey the same information as conveyed in FIG. 5. In FIG. 5, a partially formed dome is shown in the circled area. Features like these are of interest because they form upside down basins that may capture oil and gas moving toward the surface. Several lines would have to be interpreted and the correct seismic event would have to be traced on each line in order to detect this feature without a time structure display. FIG. 5 provides useful information that has previously required tedious study of many cross sections over extended periods of time. The present invention provides a far more efficient manner of obtaining such information.